When gas and/or oil is removed from a subterranean formation through drilling, the pressure in the formation declines. As a result of the pressure decline, the production of oil and/or gas from the well also declines. Although oil and/or gas may still be present in the formation, production may decrease to such an extent that further removal of oil and/or gas becomes uneconomical. This is particularly true where the cost of producing the oil and/or gas from the formation is very high, such as it is in offshore production operations. In such cases, it is crucial to extract as much oil and/or gas from the formation as possible. Production from formations in which the pressure has declined may be raised by techniques generally known as well stimulation. To stimulate an oil and/or gas well, a fracturing fluid is injected into the formation, under high pressure, via the well's bore hole. By applying hydraulic pressure within the formation, the fracturing fluid fractures the subterranean strata surrounding the bore hole, thereby increasing and extending the area from which oil and/or gas may be drained.
The most efficient hydraulic fracturing of a subterranean formation ideally requires that the fracturing fluid reach its maximum viscosity just as it enters the fracture. Increasing the viscosity of the fracturing fluid, which improves its ability to fracture the formation rock, is usually accomplished by crosslinking a water-soluble polymer (such as guar, hydroxypropyl guar, carboxymethyl guar, or carboxymethyl hydroxypropyl guar) solution. The crosslinkers used for this purpose are typically selected from the group of elements comprising aluminum, boron, and the transition metals. Currently, boron and zirconium are the elements most preferred for crosslinking aqueous solutions of water-soluble polymers.
A number of boron compounds have historically been used to crosslink the polymers employed in fracturing fluids, but there are a number of disadvantages inherent in the use of each of the currently available boron crosslinkers; including, for example, (1) some are limited to use only in low temperature (e.g., less than about 200° F. bottom-hole static temperature) applications: (2) some are difficult to control with respect to their crosslinking reaction rate, (3) some possess constituents (such as the divalent hardness ions) that are incompatible with optimum fracturing fluid performance; (4) some have poor “shelf life” and/or present other handling difficulties during well treatment; (5) some are typically very dilute (in some cases with boron concentrations a mere 10% of those of the boron crosslinker of the present invention); (6) some present environmental, health, and/or safety concerns (such as those that contain methanol or diesel as a diluent, and as such are toxic, flammable, or both); and (7) some are extremely costly and difficult to manufacture.
In addition, certain boron-containing aqueous solutions used in crosslinking systems are made at the job site by mixing solid boron-containing compounds with water or another aqueous fluid. A number of disadvantages are inherent in job-site mixing procedures, particularly when well treatment requires that large volumes of solutions be prepared. For example, special mixing equipment for mixing the solids with water is required, and extended preparation and mixing time are involved. In addition, the mixing and physical handling of large quantities of dry chemicals require a great deal of manpower, and where continuous mixing is required, the accurate and efficient handling of chemicals is extremely difficult. Thus, the use of job-site prepared aqueous boron-containing crosslinkers drives up labor costs. Moreover, uneven mixing and the low solubility of the boron solids may allow or even cause lumps to form in the crosslinker fluid, which lowers the effective concentration of the cross-linker. The time and labor necessary to assure complete solubilization and dispersion of the boron compounds used in job-site prepared crosslinkers not only further drives up costs, but also increases the likelihood that the fracturing fluid will perform poorly, or not at all.
The job-site preparation of aqueous boron-containing solutions from solids also generates fugitive dust, and the presence of such dust creates the potential for its inhalation by workers preparing the solution. The dust may produce a respiratory allergenic response and/or irritation in some individuals. Fugitive dust also raises environmental concerns.
The problems associated with mixing solids at the job site present such substantial concerns as to effectively prevent many offshore operators from using such systems. Nonetheless, because boron crosslinkers are so effective in raising the viscosity of water-soluble polymer-based fracturing fluids, there remains a desire and a need to use such crosslinkers.
In an effort to take advantage of the efficiency of boron crosslinkers while avoiding the cost, safety and environmental problems associated with their job site preparation, liquid slurries or suspensions of boron-containing solids have been used. Such suspensions are then added to the aqueous polymer systems. A number of methods for accomplishing this, and the compositions prepared thereby, are described in the prior art. However, such methods present significant disadvantages. Specifically, known slurry and suspension methods use oil carriers (e.g., mineral, isoparaffin or diesel, or hydrocarbon solvents, such as mineral spirits) to suspend and deliver the boron compounds to the aqueous systems. Examples of suspension systems utilizing mineral spirits are disclosed in U.S. Pat. Nos. 5,488,083 and 5,565,513. Recent regulations promulgated by the Environmental Protection Agency, however, limit the amount of oil or grease that can be used in well treatment fluids for offshore oilfield applications. Current regulations limit the oil and grease to a daily maximum concentration of 42 mg/l and a monthly average of 29 mg/l when the suspension is diluted to the intended use level with fresh or salt water The suspension of the present invention has hexane extractable oil and grease of less than about 30 ppm. Many of the currently available slurry and suspension systems, upon being diluted to the concentrations intended for use in fresh or salt water for a well treatment fluid, contain a much higher concentration of oil and/or grease than is permitted by the current Regulations. Thus, many of the available slurry and suspension systems are useless in offshore operations. Moreover, regulatory limits are likely to decrease, rather than increase, in the future. It is also well known that many of the oil carriers have a Pensky Martens closed cup flash point of about 140° F. or less, and as such may be classified as combustible. Pensky Martens closed cup flash point method (“PMCC”) is discussed in 29 CFR 1910.1450 and further provides a definition for “combustible liquid” as “any liquid having a flashpoint at or above 100° F. (37.8° C.), but below 200° F. (93.3° C.), except any mixture having components with flashpoints of 200° F. (93.3° C.), or higher, the total volume of which make up 99% or more of the total volume of the mixture.” Moreover, 29 CFR 1910.1450 defines a flash point as “the minimum temperature at which a liquid gives off a vapor in sufficient concentration to ignite when tested as follows: . . . (ii) Pensky Martens Closed Tester (see American National Standard Method of Test for Flash Point by Pensky Martens Closed Tester, Z117.1979 (ASTM D9379)) for liquids with a viscosity equal to or greater than 45 SUS at 100° F. (37.8° C.), or that contain suspended solids, or that have a tendency to form a surface film under test.”
In addition to the foregoing, because of the relatively low solubility of boron compounds in water, aqueous boron solutions used as crosslinkers may comprise a significant percentage (up to about 10%) of the total crosslinked fluid volume. The need for such high crosslinker volumes increases treatment costs by requiring additional equipment to transport and hold the crosslinker at the well-site. Further, such comparatively high volumes for the crosslinker additive potentially increases production costs by increasing the quantity of spent-fracturing fluid that must be recovered after the well is treated and before it can be made to produce oil or gas.
As discussed above, the currently available boron crosslinking systems present cost, health, safety, and/or environmental concerns. However, production unreliability is also a major drawback of the current systems. For example, the systems disclosed in U.S. Pat. Nos. 5,488,083 and 5,565,513 are extremely water sensitive, in that the crosslinker forms a granular precipitate or sludge in the presence of small amounts of water. This makes the crosslinker difficult-to-impossible to handle in the field, due to the potential for contamination from pumps or lines that have had water in them. While such systems have limited viability in land-based operations, they are virtually useless for offshore applications.
In addition to the environmental and water-compatibility concerns, known crosslinker suspension or slurry systems are not sufficiently stable for long term storage. That is, the boron-containing particles separate from the suspensions and settle in as little as about two to four weeks. In such time periods, the settled particles form a hard pack which either cannot be re-suspended or can only be re-suspended with extensive mixing. The inability to store such suspensions for periods longer than about two weeks presents scheduling and/or waste problems, particularly in difficult-to-reach areas, such as offshore drilling platforms.
Yet another treatment concern in using any of the currently available boron-containing crosslinking systems is the possibility of premature gelation while the fracturing fluid is in the bore hole. Premature gelation can result in poor performance, thereby further increasing treatment costs. Numerous systems that delay gelation have been developed to address the problem of premature gelation, but such delayed boron crosslinkers may be too effective for use in cold-water and/or cold-treatment applications, where the onset of crosslinking may occur substantially later than desired, i.e., after the fracturing fluid has entered the formation.
For example, extended delay times are a particular problem in applications where the fracturing fluid make-up water is colder than about 50° F., such as during the winter and in offshore treatment operations. In such circumstances, the crosslink delay time will become much longer than the well treatment will tolerate. Therefore, a need also exists for accelerated crosslinking when (1) there is a need to use cold field make-up water during cold seasons or in cold climates, (2) the wells to be treated are shallow, or (3) the fracturing fluid is required to crosslink in cold well bores, such as those commonly encountered in offshore operations.
As the availability of onshore oil supplies, particularly those located in the Middle East, becomes increasingly unsure, the ability to maximize domestic production from offshore and remote, cold region production operations will become increasingly critical to a continuous oil and gas supply. There therefore exists a need for a boron-containing crosslinking suspension system which meets the current regulatory limits for oil and grease concentrations, which is stable for long periods of time, and in which the crosslinking reaction rate may be readily accelerated in cold conditions. The present invention is directed to satisfying these needs and to overcoming the problems encountered with the boron crosslinker suspensions currently available, as discussed above.